Influence of resource availability on the foraging behavior of traplining hummingbirds in Monteverde, Costa Rica Michelle Levesque Department of Biology, University of Puget Sound ABSTRACT Traplining hummingbirds have been observed to change their foraging behavior in response to a change in resource availability. Generally, trapliners change to territorial beha vior when presented with an economically defendable resource. I investigated the degree to which aggressive behavior increases by observing changes in behavior frequency in response to increased resources. I observed that trapliner species showed an increase in aggressive behavior and a decrease in the total frequency of trap lining behavior following th e introduction of increased resources. My study also supports that traplining hummingbird s could switch their foraging behavior to territoriality when resources become economically defendable. RESUMEN Se han observado colibres que cambian su conducta de alim entacion como respuesta al cambio de la existencia de recursos. En general, los ermitanos cambian su conducta al territorialismo cuando tienen un recurso que vale la pena defender. Yo investigu el aumento de conducta agresiva al observar los cambios en la frecuencia de conducto antes y despues de aumentar los recursos. Yo observ que los especies de ermitanos aumentaron su conducto agresiva, y que todos los especies disminuyeron su conducta rutera despues de que aument la cantidad de recursos. Los resultados de esta investigacin indican que los colibres ermitanos pueden cambiar su conducto de alimentacion al territorialismo cuando vale la pena defender los recursos. INTRODUCTION Hummingbirds (family Trochilidae) are importa nt pollinators for many species of plants, particularly in the tropics, where they play an important role as faci litators of outcrossing between plant populations. They also help main tain genetic variability among neighboring plant populations (Feinsinger 1978) th rough their movement between different areas (Buono 2005) and their far-reaching flight capabilities. Several types of foraging behaviors have been observed among the species of hummingbirds found in Co sta Rica, the two most prominent being territoriality and tra plining. Territoriality is a foragi ng strategy typically observed among the more aggressive species, such as the Purp le-throated Mountaingem (Stiles and Skutch 1989). These birds fiercely defend an area of relatively dense resources against all competitors, conspecific or otherwise. The economic defendabili ty of a resource (the value of defending the resource) depends on three main factors: resour ce quality and spatial distribution, temporal distribution of the resource, and competition fo r the resource (Davies and Houston 1984). In the case of territorial hummingbirds, it is more en ergetically efficient to vigorously defend an abundant resource as long as the benefits of the resource sufficiently ou tweigh the cost of its 1
defense. Traplining, on the other hand, is used pr incipally by less-aggressive species that follow a specific route and visit multiple locations of different resources (e.g. Violet Sabrewings). The sites along the trapline are typical ly not adequate to justify defense as a primary resource. Therefore, the hummingbirds must visit more s ites and expend more energy in foraging, which is ultimately the more energetically efficient option, as apposed to defending such a small territory. If the resource richness along the trapline changes to include higher densities (as in flowering peaks), some species will switch behaviors from traplining to territoriality (Fogden and Fogden 2005). However, the conditions under which behavi or switching occurs are currently unclear. A community-wide switch to territorial behavi or could have seriously detrimental effects on the genetic health and variability of local plant populations. If a highly rich resource developed that allowed the local guild of hummingbirds to concen trate their foraging on a small territory, the plants contributing the resource could suffer from hi gh rates of self-pollination. The pollination of heterospecific plants flowering simultaneously suffers due to the high rate of pollen loss that occurs when hummingbirds move indiscriminately between multiple species (Feinsinger 1978, Feinsinger and Tiebout 1991). Howe ver, studies indicate that many plants have evolved flowering peaks at different times to avoid competition for pollinators (Feinsinger 1978, Stiles 1977) and potential hybridization. Behavior switching may also have significant consequences for pollination in terms of the reorganization of a local nectarivorous bird guild. One study observed that the dominance of one territorial hummingbird species was responsible in large part, for determining the roles and foraging patterns of all other species in th e study community (Feinsinger 1976). Subsequent changes in resource availability led to changes in the behavior of the dominant species, which then interfered with the foraging patterns of other nonter ritorial species. Significant fluctuations in resource availability could have important implications for pollina tion and outcrossing. The aim of this study is to determine the de gree to which aggressive behavior changes (and under what conditions behavior switching occurs) by simula ting a flowering peak through increasing resource availability. I hypothesize th at traplining hummingbi rds will switch their behavior to territoriality when presented with an economically de fendable resource. I predict that the amount of aggressive behavior (Table 1) will be highest in the treatmen t that has the greatest quantity of resources due to the pr obable highest benefit cost factor. I also predict that, in this treatment, the number of feeders vi sited along a traplin e will decrease. METHODS STUDY SITE AND TREATMENTS I performed this study at the Estacin Bi olgica de Monteverde in an open clearing surrounded by forest edge. I chose four sites where I could easily hang multiple hummingbird feeders, with each feeder at l east one meter below the supporting br anch and at least one and one half meters above the ground. Each of the four site s was at least ten meters from the others, and arranged to be visible from a peripheral locatio n (also at least ten meters from each site). For the first treatment, one feeder was hung at each study site, f illed with a 25% sugar solution (methods modified from McMahon 2005). The hummingbirds were given three days to discover the study site and to adjust th eir behavior before recording data. For the second treatment, I added two feeder s to each site, spaced at least one meter apart. The hummingbirds were allowed adapt to the new setup for two days before recording data. 2
DATA COLLECTION Prior to observation, I predetermined seve n behavior types (Table 1). From my observation point, I recorded behavior at each feeder site simu ltaneously for up to two hours per day. All observation periods took place between 8 am and 12 pm from October 25, 2007 to November 11, 2007. I recorded data for nine total hours during the first treat ment and eight total hours during the second treatment. At each site I recorded the number of each type of behavior per species (male and female combined). Observed species and feeding classification are listed in Table 2. If I was unable to identify an individual, I recorded the species as Unknown and included all unknown values in the territorial category of my an alysis because all behaviors were typically aggressive during the first treatment, indicating a territorial species. STATISTICAL ANALYSIS For each treatment and type of behavior, I gr ouped all data together based on the feeding classification of each hummingbird species (territorial or trapli ning), combining all data from each site. I then standardized the data from each day into measurements of behavior per hour. I used a Chi-squared Goodness of Fit test to compare values between Treatments 1 and 2 for both feeding classifications for each type of behavior. RESULTS I observed a total of seven hummi ngbird species, three consid ered to be territorial and four considered to be trapliners (Table 2). In general, the behavior of traplining hu mmingbirds differed between treatments more often than that of territorial hummingbirds. Activity also generally increased from Treatment 1 to Treatment 2 in all observed behavior types. The recorded frequency of feeding behavior in traplining species increased between Treatments 1 and 2, but was not diffe rent for territorial species in Treatment 2 (Fig. 1). The same tr end occurred for perching behavior (Fig. 2). However, both hummingbird classifications (traplining and te rritorial) showed increased frequencies of chasing, calling, and hovering behaviors (Figs. 35) between Treatments 1 and 2. The confronting behavior in trap lining hummingbirds decreased between the two treatments, but territorial hummingbirds did not differ in confront ation from the first to the second treatment (Fig. 6). For total traplining behavior, fewer feeders were visited on a trapline in Treatment 2 (Fig. 7). DISCUSSION The purpose of this study was to observe the degree to which a ggressive behavior changes due to an increase in resource availabil ity. As predicted, the data indicate more frequent instances of aggression in the treatment with higher resource availability (Fig s. 2-6). This is most likely due to the higher benefitcost factor inhe rent in greater resour ce availability. A study on competition in caged hummingbirds found that incr eased aggressive behavior led to higher energetic success in terms of the maintenance of constant energy stores and body mass over 24 hours (Tiebout 1993). For this reason, hummingb irds could gain an energetic advantage by increasing aggressive behavior when resour ces are more abundant, as in Treatment 2. 3
Also as predicted, the total number of feeders visited along traplines per hour decreased from the first to the second treatment. Such a reduction of traplining behavior in a natural setting could severely alter the pollination of plan t species, due to increased self-pollination, hybridization, and from the disrupt ion of foraging strategies in traplining species (Feinsinger 1976 and 1978, Stiles 1977). Although confronting behavior by trap lining hummingbirds decreased between Treatments 1 and 2, this could be due to a higher rate of success at chasing away competitors. Confrontations were defined as an aggressive in teraction in which one individual did not succeed in chasing the other away (Table 1). But if the trapliners were more successful at chasing in the second treatment, this would cause a decrease in confrontations and an in crease in frequency of chasing behavior (Figs. 6 and 3). The increase in the frequency of aggressive behavior of traplining species combined with the decrease in the number of feeders visited on traplines suggests that traplining hummingbirds may switch their behavior to territoriality when presented with an economically defendable resource. Despite the increased competition and energy expended from its defense, the benefits associated with greater spatial distribution of the resource outweigh th e costs of aggressive behavior. Although the data indicate a change towards te rritorial behavior in traplining species as resources increased, a future study might investigate the change be tween three different treatments (the third treatment might include fi ve feeders per site). While the hummingbirds may exhibit increased territoriality between the second and third tr eatments, they may conversely decrease their aggressive beha vior in response to richness of the resource (McMahon 2005). Another possibility for future study would be to observe traplining be havior in response to varying nectar concentrations It would be interesting to compare the changes in foraging behavior based on fluctuating spatial distribution (number of feed ers, this study) and resource richness (nectar concentration). ACKNOWLEDGMENTS Many people contributed greatly to the success of this project, and for that I owe them my deepest gratitude. I would like to thank the Estacin Biolgica de Monteverde, Tania Ch avarra for all her avian and statistical savvy, Alan and Karen Masters for their enthusiasm and refinement of my study, Pablo Allen and Taegan McMahon for answering my endless questions, and my wonderful classmates for their continuous support and for making me laugh every day. LITERATURE CITED Buono, A. E. 2005. Hummingbird facilitated polle n flow between transformed neotropical habitats. Tropical Biology and Conservation, CIEE. Fall. 87-100. Davies, N. B. and A. I. Houston. 1984. Territory economics. In J. R. Krebs and N. B. Davies (Eds.). Behavioural ecologyan evolutionary approach, pp. 148-169. Blackwell Scientific Publications, Oxford, England. Feinsinger, P. 1976. Organization of a tropical guild of nect arivorous birds. Ecological Monographs 46: 257-291. 4
---------. 1978. Ecological intera ctions between plants and hummingbirds in a successional tropical community. Ecological Monographs 48: 269-287. Feinsinger, P. and H. M. Tiebout. 1991. Co mpetition among plants sharing hummingbird pollinators: laboratory experiments on a mechanism. Ecology 72: 1946-1952. Fogden, M. and P. Fogden. 2005. Hummingbirds of Co sta Rica. Zona Tropical, Miami, Florida. McMahon, T. A. 2005. Increased agonistic behavior in hummingbirds (family Trochilidae) in Monteverde, Costa Rica with a reduction of f ood at artificial feeder s. Tropical Biology and Conservation, CIEE. Fall. pp. 56-67. Stiles, F. G. 1977. Coadapted competitors: the flowering seasons of hummingbird-pollinated plants in a tropical forest. Science 198 : 1177-1178. Stiles, G. and A. Skutch. 1989. A guide to the birds of Costa Rica, pp. 208-231 and plates 23-25. Comstock Publishing Associates, Ithaca, New York. Tiebout, H. M. 1993. Mechanisms of competition in tropical hummingbirds: metabolic costs for losers and winners. Ecology 74: 405-418. TABLE 1. A summary of standardized behaviors recorded at each site. Behavior Description 5
Feeding Each time an individual visits a feeder and feeds, re gardless of how many times the bird inserts its beak into the feeder. Perching* Each time an individu al perches near (within 3 mete rs of) the feeder site, for any length of time. Chasing* When one individual chases a nother away from the feeder site. Calling* Each time an individual calls, regardless of the duration of the calling sequence. Hover* When an individual hovers around a feed er but does not insert its beak. Also when one individual hovers around a feedi ng individual, but the latter does not respond. Confront* When two individuals engage in aggressive confrontations, but neither actually chases the other away for at least three seconds. Traplining When an individual consecutively fe eds at two or more feeders (recorded all species together and analyzed a ll numbers as Total Traplining) *Denotes aggressive behavior TABLE 2. Observed species and feeding type classification. Common Name Scientific Name Feeding Behavior Coppery-headed Emerald Elvira cupreiceps Trapliner Green Violet-ear Colibri thalassinus Trapliner Magenta-throated Woodstar Calliphlox bryantae Territorial Purple-throated Mountain-gem Lampornis ( castaneoventris ) calolaema Territorial Ruby-throated Hummingbird Archilochus colubris Trapliner Striped-tailed Hummingbird Eupherusa eximia Territorial Violet Sabrewing Campylopterus hemileucurus Trapliner 6
0 50 100 150 200 250 300 Traplining Territorial Feeding Type Treatment 1 Treatment 2 FIGURE 1. Instances of feeding for traplining species increased significantly between Treatments 1 and 2 (X2 = 12.17, df = 1, P = 0.0005). Trends for territorial spec ies indicate an increase in the number of beha vior from Treatment 1 to 2 (X2 = 0.022, df = 1, P = 0.881), though not significant. 7
0 20 40 60 80 100 120 140 160 Traplining Territorial Feeding Type Treatment 1 Treatment 2______________________________________________________________________________ FIGURE 2. Traplining species exhib ited a significant increase in perching from Treatment 1 to Treatment 2 (X2 = 69.90, df = 1, P < 0.0001). The observed numbers of perching behavior per hour did not differ significantly between treatments for territorial hummingbirds (X2 = 3.83, df = 1, P = 0.05). 0 10 20 30 40 50 60 70 80 90 100 Traplining Territorial Feeding Type Treatment 1 Treatment 2 ______________________________________________________________________________ FIGURE 3. The recorded instances of chasing be havior per hour increased significantly between treatments for both traplining (X2 = 26.89, df = 1, P < 0.0001) and territorial (X2 = 17.74, df = 1, P < 0.0001) species of hummingbirds. 8
0 20 40 60 80 100 120 140 160 180 Traplining Territorial Feeding Type Treatment 1 Treatment 2 ______________________________________________________________________________ FIGURE 4. The observed calling beha vior per hour increased significantly between Treatments 1 and 2 for both traplining (X2 = 58.76, df = 1, P < 0.0001) and territorial (X2 = 25.33, df = 1, P < 0.0001) feeding classifications. 0 10 20 30 40 50 60 70 Trapliningl Territorial Feeding Type Treatment 1 Treatment 2 ______________________________________________________________________________ FIGURE 5. The recorded numbers of hovering per hour increased significantly from Treatment 1 to Treatment 2 for both traplining (X2 = 5.62, df = 1, P = 0.018) and territorial (X2 = 10.18, df = 1, P = 0.0014) species of hummingbirds. 9
0 5 10 15 20 25 30 Traplining Territorial Feeding Type Treatment 1 Treatment 2 ______________________________________________________________________________ FIGURE 6. Confronting behavior in traplining species decreased significantly from Treatment 1 to Treatment 2 (X2 = 6.22, df = 1, P = 0.013). Conf ronting behavior per hour in territorial species did not differ significantly between treatments (X2 = 0.453, df = 1, P = 0.501). 0 5 10 15 20 25 30 12 Treatment ______________________________________________________________________________ FIGURE 7. The total number of f eeders visited (by both traplining and territorial species) on a trapline per hour decr eased significantly from Treatment 1 to Treatment 2 (X2 = 5.26, df = 1, P = 0.022). 10
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La influencia de la disponibilidad de recursos en la conducta de alimentacin de los colibres ermitaos en Monteverde, Costa Rica
Influence of resource availability on the foraging behavior of traplining hummingbirds in Monteverde, Costa Rica
Traplining hummingbirds have been observed to change their foraging behavior in response to a change in resource availability. Generally, trapliners change to territorial behavior when presented with an economically defendable resource. I investigated the degree to which aggressive behavior increases by observing changes in behavior frequency in response to increased resources. I observed that trapliner species showed an increase in aggressive behavior and a decrease in the total frequency of traplining behavior following the introduction of increased resources. My study also supports that traplining hummingbirds could switch their foraging behavior to territoriality when resources become economically defendable.
Se han observado colibres que cambian su conducta de alimentacin como respuesta al cambio de la existencia de recursos. En general, los ermitaos cambian su conducta al territorialismo cuando tienen un recurso que vale la pena defender. Yo investigu el aumento de conducta agresiva al observar los cambios en la frecuencia de conducta antes y despus de aumentar los recursos.
Text in English.
Tropical Ecology 2007
Ecologa Tropical 2007
t Monteverde Institute : Tropical Ecology